This application claims priority based on German Utility Model No. 198 49 554.4, filed Oct. 27, 1998, the entire disclosure of which is incorporated herein by reference.
The invention generally relates to position sensors such as displacement sensors and angle sensors and, particularly, to a method and apparatus for determining the absolute position of such sensors.
DE 195 06 938 discloses a method and device for measuring the angle of a rotatable body. Two individual sensors that are mechanically coupled are used. The mechanical coupling is realized by means of gears, where the number of gear teeth differ by one. Both sensors deliver a periodic signal. Optical, magnetic, capacitive, inductive or resistive sensors, i.e., contacting or non-contacting sensors, may be considered for this method.
In this known method, the difference between the measured values of each sensor multiplied by the respective number of teeth is calculated, and this value is standardized to the periodicity of the sensors. The measured angle is then determined by taking another difference, and it is determined whether the angle is negative. If it is, one complete angle period is added.
DE 196 32 656 A1 describes a method and a device for the non-contacting measurement of the position or rotational position of an object that contains two parallel tracks with magnetized increments, with the number of increments per track preferably differing by one. One sensor that respectively generates a sinusoidal or cosinusoidal output signal, depending on the relative position between the sensor and the respective increment of the track, is assigned to each track. This publication also discusses the fact that the phase difference between the angles of the sinusoidal signals of each track results in a linear signal that is piecewise positive or negative. If this signal is negative, a constant value (of 2xcfx80) is added to the difference signal.
DE 42 17 498 C2 describes an angle sensor with two tracks that have different divisions. For example, 1024 periods of markings per full circle are provided on one track and 1037 periods of markings per full circle are provided on the other track. Each track delivers both a sinusoidal and a cosinusoidal signal. Angle values for both tracks are calculated from the are tangent of the sinusoidal and the cosinusoidal signals of both tracks, with the difference of both angle values then being determined. The integral portion is then formed from this difference, with the integral portion resulting in an approximate angle value after it is multiplied by a factor. An accurate angle value is obtained by multiplying the angle value of one track by a factor. A high-resolution, absolute angle value is then obtained by adding the approximate angle value and the accurate angle value.
The invention meets the above needs and overcomes the deficiencies of the prior art by providing an improved method and apparatus of determining the absolute position of displacement and angle sensors. Among the several objects and features of the present invention may be noted the provision of such method and apparatus that permits a very precise, linear output signal even if the sensors do not deliver exactly linear output signals; the provision of such method and apparatus that prevents adding linearity errors of the sensors; the provision of such method and apparatus and the provision of such method and apparatus that is economically feasible and commercially practical.
Briefly described, a method embodying aspects of the invention determines the absolute position of two mechanically coupled sensors. The first sensor generates an output signal with a first number of periods and the second sensor generates an output signal with a second number of periods within the measuring range. The two numbers of periods differ by one. In a first step, a difference signal is formed from the output signals of the two sensors. If the difference signal is negative, the method adds a constant value to the difference signal to form a corrected difference signal. The method also includes multiplying the corrected difference signal by the first period number and then dividing it by the measuring range. After rounding this value to the next lower integer, the method multiplies the integer by the to measuring range to form a period number signal. The method further includes the steps of adding the output signal of one sensor to this period number signal to form a highly accurate, absolute output signal and forming an auxiliary signal in the form of the difference between the absolute output signal and the corrected difference signal multiplied by the period number of this sensor. The method then determines whether this auxiliary signal lies within a predetermined limiting range and further corrects the corrected, absolute output signal by adding or subtracting a predetermined value if the auxiliary signal lies outside the limiting range.
In another embodiment, an apparatus determines the absolute position of two mechanically coupled sensors. The sensors generate essentially linear, sawtooth output signals having a different number of periods within the measuring range. The number of periods of each sensor differs by one from each other. The apparatus includes an evaluation circuit having a subtracting element, a comparator and an adding element. The subtracting element forms a difference signal from the two output signals of the sensors. The comparator then determines whether the difference signal is negative. Depending on the comparison, the adding element adds a constant value to the difference signal to form a corrected difference signal. A computation component derives an integer value from the corrected difference signal multiplied by the period number of one sensor and divided by the measuring range. The computation component then multiplies this integer value by the measuring range to obtain a period number signal. The apparatus also includes another adding element for adding the period number signal and the output signal of one sensor to form a highly accurate, absolute output signal. Another computation component forms an auxiliary signal in the form of the difference between the absolute output signal minus the product of the period number of one sensor and the corrected difference signal. The apparatus further includes a window discriminator that determines whether the auxiliary signal lies within a predetermined limiting range and a third computation component that alters the absolute output signal by a correction value depending on the result of the window discriminator. This forms an absolute output signal that is free of errors.
Alternatively, the invention may comprise various other methods and systems.
Other objects and features will be in part apparent and in part pointed out hereinafter.